Radio frequency identification (“RFID”) systems have become very popular in a great number of applications. A typical RFID system 100 is shown in FIG. 1. The RFID system 100 includes an application system 110, a reader 120, and a tag 130. When the tag 130 appears in the operational range of the reader 120, it starts receiving both energy 140 and data 150 via its antenna 133 from the reader 120 via its transmitter/receiver 121 and antenna 123. A rectify circuit 131 in the tag 130 collects and stores the energy 140 for powering the other circuits (e.g., control/modulator 132) in the tag 130. After collecting enough energy 140, the tag 130 may operate and send back pre-stored data to the reader 120. The reader 120 then passes the received response data via a communications interface 160 to the server system/database 111 of the application system 110 for system applications.
The tags 130 in RFID system 100 may be classified into passive and active types according to the power provisions of the tags. Passive tags do not have their own power supply and therefore draw all power required from the reader 120 by electromagnetic energy received via the tag's antenna 133. In contrast, active tags incorporate a battery which supplies all or part of the power required for their operation.
A typical transmission method of energy 140 and data 150 between a reader 120 and a tag 130 in a RFID system 100 is by way of backscatter coupling (or backscattering). The antenna 123 of the reader 120 couples energy 140 to the tag 130. By modulating the reflection coefficient of the tag's antenna 133, data 150 may be transmitted between the tag 130 and the reader 120. Backscattering, as shown in FIG. 2, is typically used in microwave band RFID systems. Power Pin is emitted from the reader's antenna 123. A small proportion of Pin is received by the tag's antenna 133 and is rectified to charge the storing capacitor in the tag 130 for serving as a power supply. After gathering enough energy, the tag 130 begins operating. A portion of the incoming power Pin is reflected by the tag's antenna 133 and returned as power Preturn. The reflection characteristics may be influenced by altering the load connected to the antenna 133. In order to transmit data 150 from the tag 130 to the reader 120, a transistor is switched on and off in time with the transmitted data stream. The magnitude of the reflected power Preturn may thus be modulated and picked up by the reader's antenna 123.
Amplitude shift keying (“ASK”) modulation is typically used in RFID systems 100. In ASK modulation, the amplitude of the carrier is switched between two states controlled by the binary transmitting code sequence. Also, in some applications, phase shift keying (“PSK”) modulation is also used. However, arbitrary complex type modulations are generally not used in current RFID backscattering systems. Here complex type modulations are ones that are normally expressed as I+jQ, where I is the in-phase component, Q is the quadrature component, and j is the square root of −1.
For reference, the beginnings of RFID use may be found as far back as World War II. See for example, Stockman H., “Communication By Means of Reflected Power,” Proc. IRE, pp. 1196-1204, October 1948. Passive and semi-passive RFID tags were used to communicate with the reader by radio frequency (“RF”) backscattering. In backscattering RFID systems, a number of tags 130 interact with a main reader device 120 as shown in FIG. 3. The reader 130 is used to: (i) power up the tags 130 via the power of the RF signal; (ii) transfer data to the tags 130; and, (iii) read information from the tags 130.
Typically, a link budget exists between the reader 120 and the tag 130. The tag 130 communicates with the reader 120 by backscattering the RF signal back to the reader 120 using either ASK or PSK modulation. One advantage of the backscattering method is that it does not need to generate an RF carrier on chip within the tag 130, thus it requires less power, less complexity, and less cost. A typical block diagram of a backscattering transmission apparatus 400 for a tag 130 is shown in FIG. 4. In FIG. 4, Zant is the impedance of the antenna 133 and Zo is a fixed impedance which is in parallel with a switch 410. The reflection coefficient Γ is given by the equation:
  Γ  =                    Z        σ            -              Z        ant                            Z        σ            +              Z        ant            
With the switch 410 on (i.e., closed), Γ=1. When the switch is off (i.e., open), Γ=0. By turning the switch 410 on and off, an ASK signal 420 is generated as shown in FIG. 4.
PSK signals may also be generated using a similar set up. This is shown in the transmission apparatus 500 illustrated in FIG. 5. Here, the reflection coefficient Γ is given by the equation:
  Γ  =                    (                              Z            i                    +                      Z            o                          )            -              Z        ant                            (                              Z            i                    +                      Z            o                          )            +              Z        ant            
Here, Zi is an impedance that is switched in as per FIG. 5. So, depending on the position of the switch 410, 510, backscattering is designed to produce either an ASK signal 420 or a PSK signal 520.
As shown in FIG. 6, using backscattering techniques, each tag 130 sends RF signals 610 on the same carrier 620 and hence overlapping the RF spectrum of other tags 130. This poses a challenge which respect to avoiding data collisions between all of the tags 130. In current systems, these collision issues are solved via the communication protocol used between the reader 120 and the tags 130.
In Thomas S., Reynolds S. Matthew, “QAM Backscatter for Passive UHF RFID Tags”, IEEE RFID, p. 210, 2010 (Thomas et al.), the generation of four quadrature amplitude modulation (“QAM”) signals was proposed in which a number of r values are switched in and out.
There are several problems with prior tag transmission apparatus. For example, systems such as that proposed by Thomas et al. are limited in the nature of signals that they can backscatter. That is, any arbitrary signal cannot be transmitted. For example, if the QAM signal is first filtered via a filter, Thomas et al.'s system cannot transmit a filtered version of the QAM signal. As another example, if the signal is simply a sine wave or a Gaussian minimum shift keying (“GMSK”) signal, Thomas et al.'s system cannot be used to transmit this signal. As a further example, Thomas et. al.'s system cannot transmit single side band signals.
A need therefore exists for an improved transmission apparatus for wireless devices (e.g., tags) in backscattered and inductively coupled radio frequency identification systems. Accordingly, a solution that addresses, at least in part, the above and other shortcomings is desired.